Two-Dimensional Stacked Composites of Self-Assembled Alkane Layers and Graphene for Transparent Gas Barrier Films with Low Permeability.

Self-assembled alkane layers are introduced between graphene layers to physically block nanometer size defects in graphene and lateral gas pathways between graphene layers. A well-defined hexatriacontane (HTC) monolayer on graphene could cover nanometer-size defects because of the flexible nature and strong intermolecular van der Waals interactions of alkane, despite the roughness of graphene. In addition, HTC multilayers between graphene layers greatly improve their adhesion. This indicates that HTC multilayers between graphene layers can effectively block the lateral pathway between graphene layers by filling open space with close-packed self-assembled alkanes. By these mechanisms, alternately stacked composites of graphene and self-assembled alkane layers greatly increase the gas-barrier property to a water vapor transmission rate (WVTR) as low as 1.2 × 10-3 g/(m2 day), whereas stacked graphene layers generally show a WVTR < 0.5 g/(m2 day). Furthermore, the self-assembled alkane layers have superior crystallinity and wide bandgap, so they have little effect on the transmittance.

Electric field controlled near-infrared high-speed electro-optic switching modulator integrated with 2D MgO.

The electro-optic effect in two-dimensional (2D) MgO nanoflakes synthesized by a microwave-assisted process is demonstrated using a designed optical fiber modulator. The guiding properties of intense core modes excited by the material cavity are modulated by the external electric field. The feasibility of 2D MgO nanoflakes as an effective electro-optic modulator and switching are experimentally verified for the first time, to the best of our knowledge. The proposed optical-fiber-based electro-optic modulator achieves a linear wavelength shift with a high sensitivity of 12.87 pm/V(77.22 nm/kV/mm, in the electric field). The results show that MgO, as a metal oxide 2D material, is a very promising material for electro-optic modulators and switching.

Carbon-nanotube-assisted nanoepitaxy of Si-doped GaN for improved performance of InGaN/GaN light-emitting diodes.

Using single-walled carbon nanotubes (SWCNTs) as nanomasks on an undoped GaN template, a significant biaxial stress relaxation was achieved in the subsequently-grown Si-doped n-GaN layer. Enhanced near band edge (NBE) emission intensity, similar free carrier concentrations, and the reduced peak width of the asymmetric (102) crystallographic plane all confirmed the suppression of threading dislocations due to the nanoepitaxial growth process. Temperature-dependent photoluminescence (PL) revealed improved internal quantum efficiency (IQE) of InGaN/GaN multi-quantum wells (MQWs) grown on this n-GaN layer. Furthermore, enhanced light output power and a remarkable reduction in efficiency droop were observed for the blue light-emitting diodes (LEDs), especially at higher injection currents. Our results emphasize the strong potential for SWCNTs as nanomasks in the heteroepitaxy of GaN-based devices without the exploitation of complicated lithography or etching processes.

Size dependence of silica nanospheres embedded in 385 nm ultraviolet light-emitting diodes on a far-field emission pattern.

We demonstrate that the use of silica nanospheres (SNs) with sizes close to the emission wavelength of light-emitting diodes (LEDs) can enhance the light output power and manipulate the far-field emission pattern. Near-ultraviolet (NUV)-LEDs grown on a patterned sapphire substrate embedded with 300 nm SNs show a three times higher light output power than that without SNs, when measured through the top side. For far-field emission measurements, the LEDs embedded with 300 nm SNs show the significant increase of front emission due to the improved crystal quality of epitaxial films as well as the increase of Mie scattering effect of SNs. These experimental results indicate the important role of the size of embedded SNs in enhancing the light output power for NUV-LEDs.

Enhanced optical output power by the silver localized surface plasmon coupling through side facets of micro-hole patterned InGaN/GaN light-emitting diodes.

Light extraction efficiency of GaN-based light emitting diodes were significantly enhanced using silver nanostructures incorporated in periodic micro-hole patterned multi quantum wells (MQWs). Our results show an enhancement of 60% in the wall-plug efficiency at an injection current of 100 mA when Ag nano-particles were deposited on side facet of MQWs passivated with SiO2. This improvement can be attributed to an increase in the spontaneous emission rate through resonance coupling between localized surface plasmons in Ag nano-particles and the excitons in MQWs.

Time-resolved ultraviolet near-field scanning optical microscope for characterizing photoluminescence lifetime of light-emitting devices.

We developed a instrument consisting of an ultraviolet (UV) near-field scanning optical microscope (NSOM) combined with time-correlated single photon counting, which allows efficient observation of temporal dynamics of near-field photoluminescence (PL) down to the sub-wavelength scale. The developed time-resolved UV NSOM system showed a spatial resolution of 110 nm and a temporal resolution of 130 ps in the optical signal. The proposed microscope system was successfully demonstrated by characterizing the near-field PL lifetime of InGaN/GaN multiple quantum wells.

Keratinocyte-derived laminin-332 promotes adhesion and migration in melanocytes and melanoma.

Melanocytes are highly motile cells that play an integral role in basic skin physiological processes such as wound healing and proper skin pigmentation. It has been postulated that surrounding keratinocytes contribute to melanocyte migration, but underlying mechanisms remain rather vague so far. In this study, we set out to analyze the specific potential contribution of keratinocyte components to melanocytes and melanoma cell migration-related processes. Our studies revealed that A375 human melanoma cell attachment, spreading, and migration are interestingly better supported by HaCaT keratinocyte extracellular matrix (ECM) than by self-derived A375 ECM. Moreover, HaCaT ECM caused increased integrin α6 expression, adhesion-mediated focal adhesion kinase phosphorylation, and focal adhesion formations. Similar effects were confirmed in human melanocytes. Furthermore, we found that keratinocyte-derived soluble factors did not appear to significantly contribute to these processes. Specific extrinsic factors that promoted melanoma migration were attributed to keratinocyte-derived laminin-332, whereas alternative ECM component such as laminin-111 and fibronectin functions appeared to have insignificant contributions. Taken together, these studies implicate extrinsic laminin-332 in promoting the high mobility property and perhaps invasiveness inherently characteristic of, and that are the menace of, melanocytes and melanomas, respectively.

Efficiency improvement in InGaN-based solar cells by indium tin oxide nano dots covered with ITO films.

InGaN based MQW solar cells have been fabricated with 4 different transparent top electrode structures: (1)- ITO 200 nm, (2)-ITO nano dots only, (3)-ITO nano dots on ITO 50 nm and (4)-ITO nano dots on ITO 100 nm. The solar cell with the ITO 50 nm on ITO nano dots under AM 1.5 conditions showed the best results: 2.3 V for V(oc), 0.69 mA/cm(2) for J(sc), 41.8% for peak EQE, and 0.91% for conversion efficiency. Efficiency improvement was possible due to the decreased reflectance achieved by the ITO nano dots covered with an ITO film with optimized thickness.

Efficiency improvement in InGaN-based solar cells by indium tin oxide nano dots covered with ITO films.

InGaN based MQW solar cells have been fabricated with 4 different transparent top electrode structures: (1)- ITO 200 nm, (2)-ITO nano dots only, (3)-ITO nano dots on ITO 50 nm and (4)-ITO nano dots on ITO 100 nm. The solar cell with the ITO 50 nm on ITO nano dots under AM 1.5 conditions showed the best results: 2.3 V for V(oc), 0.69 mA/cm(2) for J(sc), 41.8% for peak EQE, and 0.91% for conversion efficiency. Efficiency improvement was possible due to the decreased reflectance achieved by the ITO nano dots covered with an ITO film with optimized thickness.

Enhancement of light output power in GaN-based light-emitting diodes using hydrothermally grown ZnO micro-walls.

We report on the efficiency enhancement in GaN-based light-emitting diodes (LEDs) using ZnO micro-walls grown by a hydrothermal method. The formation of ZnO micro-walls at the indium tin oxide (ITO) border on the LED structure is explained by the heterogeneous nucleation effect. The light output power of LEDs with ZnO micro-walls operated at 20 mA was found to increase by approximately 30% compared to conventional LEDs. Moreover, the finding of nearly the same current-voltage characteristics of GaN-based LEDs with and without a ZnO micro-wall shows that the ZnO micro-wall does not influence the electrical properties of the device but only leads to an increase in the light extraction efficiency. From the confocal scanning electroluminescence results, we confirm that ZnO micro-walls enhance the light output power via the photon wave-guiding effect.

p63 protects the female germ line during meiotic arrest.

Meiosis in the female germ line of mammals is distinguished by a prolonged arrest in prophase of meiosis I between homologous chromosome recombination and ovulation. How DNA damage is detected in these arrested oocytes is poorly understood, but it is variably thought to involve p53, a central tumour suppressor in mammals. While the function of p53 in monitoring the genome of somatic cells is clear, a consensus for the importance of p53 for germ line integrity has yet to emerge. Here we show that the p53 homologue p63 (refs 5, 6), and specifically the TAp63 isoform, is constitutively expressed in female germ cells during meiotic arrest and is essential in a process of DNA damage-induced oocyte death not involving p53. We also show that DNA damage induces both the phosphorylation of p63 and its binding to p53 cognate DNA sites and that these events are linked to oocyte death. Our data support a model whereby p63 is the primordial member of the p53 family and acts in a conserved process of monitoring the integrity of the female germ line, whereas the functions of p53 are restricted to vertebrate somatic cells for tumour suppression. These findings have implications for understanding female germ line fidelity, the regulation of fertility and the evolution of tumour suppressor mechanisms.

Defect-selective-etched porous GaN as a buffer layer for high efficiency InGaN/GaN light-emitting diodes.

Substrate-induced biaxial compressive stress and threading dislocations (TDs) have been recognized to severely impair the performance, stability, and reliability of InGaN/GaN light-emitting diodes (LEDs) for quite some time. In this study, a defect-selective-etched (DSE) porous GaN layer is fabricated employing electro-chemical etching and applied as a buffer layer for the development of InGaN/GaN LEDs with high quantum efficiency. Based on the analysis of photoluminescence and micro-Raman spectra, it has been revealed that the overgrown GaN epilayer on the DSE porous GaN has a relatively low TDs and relaxation of compressive stress in comparison to the conventional GaN epilayer. The remarkable improvement in the internal quantum efficiency of the InGaN/GaN LEDs is directly attributable to the strong radiative recombination in InGaN/GaN multi-quantum-wells caused by stress relaxation and TDs annihilation. Our findings indicate that the use of DSE porous GaN as a buffer layer may be a viable approach for producing crystalline GaN epilayers and high-performance LEDs.

Light outcoupling effect in GaN light-emitting diodes via convex microstructures monolithically fabricated on sapphire substrate.

GaN-based light-emitting diode (LED) was fabricated on the sapphire substrate with monolithic convex microstructures (CMs) array. Using confocal scanning electroluminescence (EL), we have directly observed the strong outcoupling phenomenon of the light confined in a LED via the CMs array. This outcoupled light could be efficiently converged on the convex center through consecutive reflections at the flat area and the curved slant area of the CMs array. Compared to the conventional LED, the ray tracing simulation and far field EL results of the LED with a CM array showed efficient light extraction toward the top surface, i.e., 0-5, 40-45 and 60-65 degree by the outcoupling effect. We conclude that the outcoupled optical path via CMs is the dominant factor of the enhanced light extraction in the LED with a CM array.

Enhanced light output power of near UV light emitting diodes with graphene / indium tin oxide nanodot nodes for transparent and current spreading electrode.

We report GaN-based near ultraviolet (UV) light emitting diode (LED) that combines indium tin oxide (ITO) nanodot nodes with two-dimensional graphene film as a UV-transparent current spreading electrode (TCSE) to give rise to excellent UV emission efficiency. The light output power of 380 nm emitting UV-LEDs with graphene film on ITO nanodot nodes as TCSE was enhanced remarkably compared to conventional TCSE. The increase of the light output power is attributed to high UV transmittance of graphene, effective current spreading and injection, and texturing effect by ITO nanodots.

Trap-state-assisted white light emission from a CdSe nanocrystal integrated hybrid light-emitting diode.

This Letter reports on the fabrication of hybrid white-light-emitting diodes made of semiconductor nanocrystals (NCs) integrated on InGaN/GaN LEDs. Using core type and core/shell type CdSe NCs, the white light properties are systematically engineered for white light generation with high color rendering index (CRI). Unlike CdSe/ZnS core/shell NCs, which exhibited a unique narrowband edge emission, core type CdSe NCs offered extended broad emission toward orange/red wavelengths associated with deep trap states. Consequently, the light-emitting properties of the devices showed strong dependence on the type of NCs used, and devices with CdSe NCs offered admirable characteristics, such as Commission Internationale d'Eclairage coordinates of (0.356, 0.330) and a CRI as high as 87.4.

Enhanced air-cavity effect of periodically oriented embedded air protrusions for high-efficiency InGaN/GaN light-emitting diodes.

We report on the development of periodically oriented embedded air protrusion (EAP) structures at the GaN-sapphire interface in InGaN/GaN LEDs. A specific SiO(2) mask pattern and a simple wet etching process were utilized for the fabrication of EAP structures. A strong coupling between closely proximate air cavities and the multiple quantum wells promoted spontaneous emission due to the high-index contrast at the GaN-air interface. As a result, the light output power of the EAP LED was 2.2 times higher than that of a conventional LED at an injection current of 20 mA.

Boron Nitride as a Passivation Capping Layer for AlGaN/GaN High Electron Mobility Transistors.

We report on the electrical characteristics of AlGaN/GaN high-electron mobility transistors (HEMTs) with hexagonal boron nitride (h-BN) as a passivation capping layer. The HEMTs with h-BN layers showed an increase in current drainage and 103-times reduction in the gate-leakage current compared with those of conventional unpassivated HEMTs. Moreover, the extrinsic transconductance and the pulse responses were improved due to the reduced charge-trapping effect at the surface of HEMTs. From our observations, the h-BN can be used as a passivation capping layer for high-power electronic devices.

The impact of ZnO nanoparticle interlayer on the growth and morphology of broom-like single crystalline gallium nitride.

Broom-like single crystalline gallium nitride (GaN) nanomaterials have been successfully grown on Si (100) substrates through ammoniating the new type of complex precursor hexafluoroammonium gallate [(NH4)3GaF6], under a constant flow of ammonia at 900 degrees C in a quartz tube. This research mainly deals with the effect of zinc oxide (ZnO) interlayer on the growth and morphological changes of GaN nanoparticles. Broom-like GaN nanomaterials were obtained upon the influence of very fine ZnO nanoparticle interlayer in between the silicon substrate and the Ga source complex materials. On the other hand, well organized micron size hexagonal GaN particles were resulted in absence of ZnO interlayer. The transformation of hexagonal to broom-like GaN nanomaterials was predominant due to improved surface compatibility between the Si substrate and Ga source complex by the ZnO interlayer. The starting point for the transformation was indentified by controlling the process time. The structure and morphology changes of the GaN product were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images demonstrate the difference in their morphological changes of single crystalline GaN upon the ZnO nanoparticles layer deposition.

Synthesis of single-crystalline ZnO nanowires and their applications in dye-sensitized solar cells (DSSCs) with a solid polyethylene glycol (PEG) redox electrolyte.

We demonstrate for the first time ZnO nanowire dye-sensitized solar cells (DSSCs) with a solid-state poly (ethylene glycol) (PEG) redox electrolyte. From the current-voltage characteristics of this solid PEG electrolyte-based ZnO nanowire DSSCs, the open-circuit voltage and short-circuit current density, and fill factor were determined to be approximately 0.58 V, approximately 1.30 mA/cm2, and 0.31, respectively. We obtained a power conversion efficiency of approximately 0.24% under an incident irradiation of 100 mW/cm2, corresponding to air mass (AM) 1.5 global solar conditions. The solid-state DSSC experienced an efficiency that was approximately 0.1% lower than the efficiency a liquid electrolyte based ZnO nanowire DSSC.

Boron nitride nanotubes as a heat sinking and stress-relaxation layer for high performance light-emitting diodes.

High-density threading dislocations, the presence of biaxial compressive strain, and heat generation are the major limitations obstructing the performance and reliability of light emitting diodes (LEDs). Herein, we demonstrate a facile epitaxial lateral overgrowth (ELOG) method by incorporating boron nitride nanotubes (BNNTs) on a sapphire substrate by spray coating to resolve the above issues. Atomic force microscopy, X-ray diffraction, micro-Raman, and photoluminescence measurements confirmed the growth of a high quality GaN epilayer on the BNNT-coated sapphire substrate with reduced threading dislocations and compressive strain owing to the ELOG process. GaN LEDs fabricated using this approach showed a significant enhancement in the internal quantum efficiency and electroluminescence intensity compared to conventional LEDs grown on sapphire. Moreover, reduced efficiency droop and surface temperature at high injection currents were achieved due to the excellent thermal stability and conductivity of BNNTs. Based on our findings we infer that the BNNTs would be a promising material for high power devices vulnerable to self-heating problems.